Detergent bars are conventionally manufactured by one of two methods; (i) milling followed by extrusion ("plodding") and stamping (sometimes referred to as the "milling" process), or (ii) casting.
In the milling process, a preformed solid composition comprising all components of the bar is typically plodded, i.e. extruded through a nozzle to form a continuous "rod" which is cut into smaller pieces of predetermined length, commonly referred to as "billets". These "billets" are then fed to a stamper or, alternatively, are given an imprint on one or more surfaces using, for example, a die of the same dimensions as the bar surface which is hit with force such as with a mallet or a die in the shape of a roller, or simply cut.
There are several shortcomings associated with the milling method of detergent bar manufacture.
A problem encountered with the stamping process is die-blocking, in which amounts of residual detergent left on die halves build up during continued use of the dies. Die blocking can lead to poor or even non-release of the bars from the die surface and/or visible imperfections on the bar surface. Extrusion and stamping also require that the extruded billet be in a substantially "rigid" form at the process conditions. Die blocking and "soft" billets may be caused by soft detergent compositions, for example compositions containing a large proportion of ingredients which are liquid at processing conditions, and/or may also be a result of the shear and extensional forces to which the detergent composition is subjected by the milling process, e.g. the extrusion and/or stamping.
Milling is therefore only suitable for formulations which are plastic and yet which are not soft or do not become soft or sticky due to the shear degradation at operating temperatures of the manufacturing equipment, typically in the range of ambient .+-.30.degree. C.
Milled bars also tend to have an oriented structure, aligned along the axis of extrusion. They also tend to form cleavage planes within the bar, which weaken the bar and, with the repeated wetting and drying of the bar in use, can lead to wet-cracking along the planes. Wet-cracking is highly undesirable being both unsightly and leading to bar fracture.
The other conventional method for the manufacture of detergent bars is casting. In casting, detergent compositions in a heated mobile and readily pourable state are introduced into the top of an enclosed cavity (i.e. a mould) of the desired shape and the temperature of the composition reduced until it solidifies. The bar can then be removed by opening the mould.
In order to be castable, the detergent formulation must be mobile and readily pourable at the elevated temperatures employed. Certain detergent formulations are viscous liquids or semi-solids at commercially realistic elevated temperatures and therefore do not lend themselves to casting.
Furthermore, in the casting process, the detergent melt tends to cool slowly and unevenly. This can lead to unwanted structural orientations and segregation of ingredients. Often some sort of active cooling system is employed in order to achieve acceptable processing times. Even when a cooling system is employed, cooling is still generally uneven through the detergent composition in the mould.
A major problem with the casting process is that detergent compositions in the moulds tend to shrink as they cool. This is highly undesirable as the mould is intended to impart a distinctive shape on the bar and/or a logo of some kind. Shrinkage can take the form of dimples, wrinkles or voids, or a depression at the fill point of the bar.
Therefore, there is a need for a process and apparatus for forming detergent compositions into good quality bars (i.e. bars, for example, of good appearance and physical characteristics) which overcomes the identified problems and disadvantages associated with the milling process, and which also avoids the problems associated with casting.
U.S. Pat. No. 2,987,484 (Procter & Gamble) discloses a closed die moulding process in which a basically non-soap fluid mixture of synthetic detergent and a binder-vehicle is rapidly injected through a small orifice into a substantially closed die, the fluid mixture being capable of solidifying into a shape-sustaining form.
The process involves heating the composition to a temperature in the range 70.degree. C. to 150.degree. C. so that the composition melt is in a fluid-injectable state. In all the examples, the temperature is in the range 82-150.degree. C. The melt is circulated through a continuous injection circuit comprising a crutcher in which the fluid mixture is mixed and heated, a pipeline in a loop with the crutcher, a heat exchanger in the pipeline to stabilise the temperature of the melt, and a pump to maintain the circulating and injection pressure.
The viscosity of the heated melt at the conditions of injection is 2-50 Pa.s. This is described as being dependent on the intensity of shear and the temperature and a function of the composition. However, no specific shear rates are given for this viscosity range. A melt having a viscosity in the range 2-50 Pa.s at injection conditions is described as being thick enough so as not to splash in the mould, entrap air or run out of the mould air vents, whilst being thin enough to permit complete filling of the mould prior to solidification of any composition therein and to avoid excessive injection pressures. Suitable injection pressures range from about 1-20 psi, but are preferably in the range 2-10 psi. In all the examples, the injection pressure is between 5-8 psi. Pressures which are too high are described as causing splashing in the mould and as increasing the density of the melt.
U.S. Pat. No. 2,987,484 also teaches, and it is an essential feature of the claims, that for the process to work, the fluid mixture must be cooled through a nigre (isotropic liquid) plus crystals phase. Furthermore, it is taught that detergent fluid mixtures in the neat or middle (anisotropic liquid) phases are not suitable for closed die moulding because of the excessive viscosity of these phases and the tendency for undesirable complexes to form in these phases. In addition, U.S. Pat. No. 2,987,484 states that successful closed die moulding necessitates avoidance of cooling through neat and middle phases (column 4, lines 8 to 27).
U.S. Pat. No. 2,989,484 is described as overcoming the problems associated with conventional methods of bar manufacture and in particular those associated with milling. However, the solution described has several inherent drawbacks, most of which are common to the casting and framing processes. It is very energy intensive, energy being required to heat the detergent compositions to the high temperatures at which the fluid mixture is injected and subsequently to cool the moulds in order to reduce the solidifying times to acceptable levels. Furthermore, by injecting the compositions as high temperature fluids, the process leads to problems with shrinkage of the bars as they solidify. It also fails to address the problem of segregation of ingredients as the detergent composition cools in the mould. The detergent composition in the apparatus is permanently sheared by being pumped through pipes or by a mixer in the crutcher.
Conventional processes of detergent bar manufacture operate either by structuring the detergent composition totally within the mould, requiring initial high heat energy input (e.g. casting), or structuring the detergent composition totally outside the mould/bar-shaping means, resulting in the processing of a rigid solid material prior to moulding (e.g. extrusion and stamping). The latter type of process subjects the structured material to high shear energy (e.g. in stamping). In attempting to overcome the shortcomings of such processes, and in particular those of the milling and framing processes, the process described in U.S. Pat. No. 2,987,484 does not deviate from this general pattern--there is a high energy input in terms of the relatively high temperatures used. From this perspective, U.S. Pat. No. 2,987,484 merely provides an alternative casting process in which the detergent material is injected, rather than being poured, into a mould.
The present inventors have found that the problems present in the methods of the prior art can be overcome by operating in a processing window whereby structure is developed partially outside and partially inside the mould. In this way, any disruptive shear effects present in the process will only act on a partially-developed structure and sufficient structure can form in the mould to produce good quality bars. In this way, the structuring of the detergent composition is damaged to a much lower degree during bar formation and higher injection pressures can be tolerated, without disrupting the partial structure.